This invention relates to liquid crystalline mixtures.
In an electric field, the molecules of liquid crystalline compounds and mixtures which possess a positive anisotropy of the dielectric constants (i.e., .epsilon..sub..parallel. &gt;.epsilon..sub..perp.) are oriented with their longitudinal axes parallel to the field direction. .epsilon..sub..parallel. signifies the dielectric constant along the longitudinal axis of the molecule and .epsilon..sub..perp. signifies the dielectric constant perpendicular thereto.
This dielectric field effect is utilized in the interaction between the liquid crystalline molecules and guest molecules (guest-host interaction) described by J. H. Heilmeier and L. A. Zanoni [Applied Physics Letters 13, 91 (1968)]. Another application of the dielectric field effect is the electro-optical rotation cell discovered by M. Schadt and W. Helfrich [Applied Physics Letters 18, (1971)]. A further example is the Kerr cell described in Molecular Crystals and Liquid Crystals 17, 355 (1972).
The above electro-optical rotation cell includes a condenser-like structure having transparent electrode plates, the dielectric of which is formed from nematic liquid crystal material with .epsilon..sub..parallel. &gt;.epsilon..sub..perp.. The longitudinal axes of the liquid crystal molecules are arranged in twisted or helical form between the plates in the fieldless state. The twisting structure is determined by the given wall orientation of the molecules. After applying an electric potential to the condenser plates, the molecules adjust themselves with their longitudinal axes in the field direction (i.e., perpendicular to the surface of the plates), so that linear polarized light no longer rotates in the dielectric (the liquid crystal is uniaxially perpendicular to the surface of the plates). After removing the electric potential, the molecules return to their prior orientation. This reversible effect on the molecules can be used to electrically control the optical transmissivity of the condenser. To achieve an optimal transition between these two orientations, the threshold potential of the compounds or mixtures is adjusted to the driving potential of the rotation cell. The driving potential of such a "light rotation cell" is dependent on the battery potential and the control circuit used.
In a rotating cell, it is desirable to use compounds or mixtures which have low threshold and operating potentials. This is particularly important when using the rotation cells in clock displays.
We have invented liquid crystalline mixtures which advantageously possess low threshold and operational potentials.